There are, in principle, various possibilities for detecting a temperature of a fluid being used in a vehicle heater, for example, by measuring an electric resistance showing a significantly temperature-dependent behavior in a relevant temperature range. Moreover, it is possible to detect the temperature of a fluid on the basis of the radiation emitted by this fluid. This measurement method is based on the Stefan-Boltzmann law, according to which the radiation output emitted by a body is proportional to the fourth power of T. However, an accurate temperature detection on the basis of this measurement method is only possible if the radiation received can be unambiguously assigned to the fluid being measured, because radiation emitted by the area surrounding the fluid may otherwise considerably distort the temperature determination. This happens especially if the fluid is a gas, because this has a substantially lower density than the surrounding area.